The present invention relates generally to organic light-emitting diode (OLED) displays, and more specifically to TFT drivers for the OLEDs.
An OLED generates light by a current flowing through an organic compound which is fluorescent or phosphorescent and excited by electron-hole recombination. OLEDs have low profile and a wide view angle. There are two types of driving modes for the OLED, namely, a passive type and an active type. The active type is more suitable for a wide-screen and provides high-resolution. Thin-film transistors (“TFTs”) are used to drive the active type of OLEDs. TFTs are made from two types of materials—poly silicon and amorphous silicon (a-Si).
A low temperature poly silicon TFT is capable of delivering a large current due to large mobility and is therefore capability of yielding a bright display. However, the poly silicon TFT requires nine photoengraving process (PEP) steps to manufacture, and therefore, is expensive to manufacture. Moreover, it is difficult to make a large screen with poly silicon TFTs, and today this is limited to about fifteen inches. On the contrary, the amorphous silicon (“a-Si”) TFT can be formed with fewer manufacturing process steps, and therefore, is less expensive. Moreover, the a-Si TFT can be formed into a large screen and has high image quality with uniform luminance.
The OLED is a current-driven element and its luminance depends on the amount of current flowing through it. Accordingly, if the driving transistors do not supply a uniform current or if this current changes with time, the resultant image will degrade. The operation of the driving transistor is also impacted by the threshold voltage of its gate. The variation of the threshold voltage for poly silicon transistors initially and over time is small, which is advantageous. However, the variation of the threshold voltage for amorphous silicon over time is substantial, and this contributes to the lack of uniformity of the drive current. One reason for the variation of threshold voltage (Vth) for both types of TFTs is that electrons jump into a gate insulating film when the electrons flow on a channel of the TFT. Also, Si is charged by the electrons upon flowing on the channel of the TFT because the electrons disconnect Si bonds.
FIG. 6 is a graph showing variation of the threshold voltage (Vth) over time of an amorphous silicon TFT. The threshold voltage increase over time from about 0.7 V at the start to about 2.0 V after ten hours of operation. For a constant drive voltage, the output current decreases as the threshold voltage (Vth) increases resulting in lower luminence of the resultant image. Also, when Vth increases, the image gray-scale degrades near the black end.
An object of the present invention is to reduce the variation over time of a threshold voltage (Vth) of a TFT or other transistor used to drive an OLED.